1. Field of the Invention
The present invention relates to a Doppler velocimeter useful for the measurement of the blood flow velocity in a blood vessel such as that on the fundus of an eyeball or the like and an eye examining apparatus, such as a fundus blood flow meter, or the like.
2. Related Background Art
U.S. Pat. Nos. 5,106,184 and 5,894,337 describe a Doppler fundus blood flow meter that uses the following measurement principle. A blood vessel to be measured is illuminated with measurement light. Two detectors receive from two directions a light beat signal obtained by mixing a scattered light signal, which has been Doppler-shifted by fine particles such as red blood cells contained in blood flowing in the blood vessel, and scattered reference light from a blood vessel wall or surrounding tissue, which has not been Doppler-shifted. These signals undergo FFT (fast Fourier transform) waveform analysis. These references assume as a blood vessel model a so-called Poiseuille flow, which has a maximum blood flow velocity at its center and a lower flow velocity toward the periphery, as shown in FIG. 18. For the two detectors, cutoff frequencies xcex94fmax1 and xcex94fmax2 as maximum Doppler shifts corresponding to the maximum blood velocity at the center of the blood vessel are obtained, as shown in FIGS. 19 and 20, and a maximum blood flow velocity is obtained from these values.
It is a common practice to visually determine this cutoff frequency by the operator. APPLIED OPTICS, Vol. 27, No. 6, pp. 1126-1134 (1988) xe2x80x9cRetinal laser Doppler velocimetry: toward its computer-assisted clinical usexe2x80x9d (B. L. Petrig, C. E. Riva) discloses a method of obtaining the cutoff frequency by assuming an ideal model in which the power spectrum of the FFT waveform vertically drops at a given cutoff frequency. However, the power spectrum of an actual FFT waveform does not have an ideal shape that vertically drops discontinuously, but has a shape that abruptly and continuously drops along a given trend curve. For this reason, it is difficult for the method disclosed in the above reference to accurately discriminate the cutoff frequency.
The present invention is a challenge to an improvement in the prior art based on the above situation and has as its principal object to provide a Doppler velocimeter which can attain measurement precision higher than the prior art, and can achieve size and cost reductions.
More specifically, it is the first object of the present invention to provide a system which adopts an automated method that can obtain the cutoff frequency with higher precision, and can accurately measure the flow velocity. It is the second object of the present invention to provide a system which can achieve high-precision measurement using a smaller number of detectors than the prior art.
In order to achieve the above objects, one aspect of a Doppler velocimeter according to the present invention comprises an illumination system for illuminating a blood vessel with a light beam, a detector for detecting Doppler-shifted scattered light produced from blood flowing in the blood vessel, and a processor for obtaining a blood flow velocity by analyzing a signal from the detector, wherein the processor compares an actual power spectrum obtained by frequency-analyzing the signal, and a theoretical power spectrum obtained using a frequency as a parameter while changing the parameter.
Other objects and aspects of the present invention will become apparent from the following description of the embodiments.